This invention relates to measuring apparatus and more particularly to apparatus for measuring the size of solid particles falling in a fluid of known viscosity.
There are a number of presently available techniques for ascertaining the dimensions of small particles. These have included, in addition to the well known sieve or screen analyses, the measurement of the particles microscopically, sedimentation techniques, permeability systems, adsorption and photo-extinction. In the oil and gas field, for example, the measurement of rock and sand particles, etc., commonly is accomplished by passing the particles through a series of screens of different sizes to obtain a rough quantification of the size of the particles retained on each screen. For particles in the less than fine sand size range or in other cases in which more rapid particle size information is desired, however, the screen analysis technique has proven deficient, and it frequently is desirable to turn to one or more of the other available measuring systems. Also, most sedimentary rocks were deposited by water, and it is highly desirable to use a closely related technique in the particle size analysis of such rocks.
Sedimentation techniques are generally more reliable and accurate than some of the other methods for determining the size of the smaller size particles. The sedimentation techniques are based in whole or in part on the well known Stokes' Law relating the velocity of the particles settling in a fluid to the size and density of the particles and the viscosity and density of the fluid. By making certain simplifying assumptions the particle size may be ascertained in a reasonably straightforward manner without the need for tedious microscopic examination of the particles or overly expensive and sophisticated equipment.
The sedimentation techniques and equipment employed heretofore, however, have exhibited a number of disadvantages. As an illustration, when using several types of prior techniques and apparatus for this purpose the particles often agglomerated or were otherwise unevenly dispersed at the time of deposition with the result that the overall accuracy of the measurements was impaired. Furthermore, and this has been of special moment in analyzing core samples and in other instances where there is a need for prompt and accurate particle size data, the amount of time required for making the measurements was excessive in many instances.